1. Field of the Invention
The present invention relates to a photosensor for use in electrophotography adopted in an apparatus such as a copying machine, a printer, a facsimile etc. which employs a copying process, for instance, of the Carlson method.
2. Brief Description of the Prior Art
FIG. 5 is a cross-sectional view for illustrating a schematic constitution of an exemplified conventional photosensor in general for use in a copying machine or in a printer which employs an electrophotographic technology. In FIG. 5, a numeric character 1 stands for an electrically conductive substrate, 2 stands for an amorphous photoconductive layer which includes silicon atoms as a major element and 3 stands for a surface protective film for protecting the photoconductive layer 2.
On the other hand, another constitution shown in FIG. 6 has been proposed as another exemplified conventional photosensor for use in the copying machine or in the printer employing the electrophotographic technology. In FIG. 6, a numerical sign 11 stands for another conductive substrate, 12 stands for an adhesion enhancement film, 13 stands for another photosensitive layer and 14 stands for another surface protective film, all of which are formed as an consecutive amorphous film 15.
The photosensor for use in electrophotography constituted as mentioned above are fabricated by decomposition of silane gas (referred to as "SiH.sub.4 ") employing a glow discharging technology. During that, an adequate amount of hydrogen gas (referred to as "H.sub.2 ") is doped to reduce a dangling bond density in the film by a termination technology using hydrogen atoms while a desired amount of boron atoms (referred to as "B") is doped to improve an electric charge retaining ability of the photosensor. As a result, a film resistivity measured in a dark space is increased to be higher than 10.sup.12 --10.sup.13 ohm-cm, which makes the photosensor adaptable to an electrophotographic process (copying process) utilizing the Carlson method.
In the conventional amorphous silicon (referred to as "a-Si") photosensor for use in electrophotography including the silicon atoms as the major element, however, there have been big problems as follows:
(A) Although the first-type conventional photosensor mentioned above provides initially an excellent image, a storage in an open air atmosphere or in a highly humid ambient for a long period of time frequently induces image failures, in particular blurrings and flowed images, while continuous repetitions of printing numerous numbers of characters result in the flowed image failures.
Origins of generating the anomalous images have been thought in general attributed to it that an outermost surface of the photosensor is chemically deteriorated, being suffered from ill effects of chemical species such as ozons, nitrogen oxidants, nascent oxygens etc. generated by corona discharging phenomena which take place during machine processings.
Many species of the surface protective films made of the a-Si compounds have been developed to prevent the image failures mentioned above from generation and simultaneously to improve a surface durability during printing. However, any protective film which can cope with all causes of the image failures has not been developed yet. (B) On the other hand, the second-type conventional photosensor mentioned above for use in electrophotography which is formed of the a-Si compounds exhibits a higher pressure durability against a force applied from an external because it has a higher hardness compared with any other photosensors. However, strong demands for acceleration in printing speed and for a full-color printing have recently diversified the printing processes themselves so that some printing processes cause flaws on a surface of the photosensor due to constituent material dependence of the protective film, which turns to be another origins of the image defects.
When the second-type conventional photosensor is used in a high-temperature and high-humidity ambient on the contrary, either the flowed images or the blurrings generate frequently. Those phenomena similarly depend much on the constituent materials and so on.
Although both aforesaid image failures originated from the flaws and above-mentioned flowed images caused from the corona discharge should be eliminated from the photosensor to be used for electrophotography, elimination of all of such defects is difficult at present from the photosensors formed of the a-Si materials, conclusively.